Just for the record, yes LGP files are exclusive to the PC version. But Im fairly confident that what youre asking is possible, because afaik, aside from a few details, such as the method of compression, the game files themselves were brought straight over from the PSX version.
Also, if Im wrong please correct me, but didnt the PSX version use LZS compression for the archives that are LGP on the PC? If so, fice has a tool for those archives on his site I believe, which would allow you to extract the hi-res model from the PC version using LGP Tools, then import it into the PSX version's LZS file, then create an ISO from that.
But dont take my word for it until someone can verify all that, because I dont own a PSX version and cant check that to be certain.